Absorption and fluorescence spectra of ring-substituted indole-3-acetic acids

The absorption and fluorescence spectra of indole-3-acetic acid (1), a plant growth regulator (auxin) and experimental cancer therapeutic, 29 ring-substituted derivatives and the 7-aza analogue (1H-pyrrolo [2,3 b]pyridine-3-acetic acid) are compared. Two to four absorbance maxima in the 260-310-nm range are interpreted as overlapping vibronic lines of the L-1(a) --> (1)A and L-1(b) --> 'A transitions. Two further maxima in the 200-230-nm region are assigned to the B-1(a) --> (1)A and B-1(b) <-- (1)A transitions. 4- and 7-Fluoroindole-3-acetic acid exhibit blue shifts with respect to 1, most other derivatives show red shifts. All indole-3-acetic acids studied, with the exception of chloro-, bromo- and 4- or 7-fluoroderivatives, fluoresce at 345-370 nm when excited at 275-280 nm. 7-Azaindole-3-acetic acid emits at 411 nm. The fluorescence quantum yield of 6-fluoroindole-3-acetic acid significantly exceeds that of 1 (0.3); the other derivatives have lower quantum yields. The plant-growth promoting activity of the ring-substituted indole-3-acetic acids studied correlates with the position of the B-1(b) --> 'A transition band. (C) 2004 Elsevier B.V. All rights reserved.